Circuit for measuring self-inductances and capacitances and changes thereof



A nl 17, 1962 w. A. J. VAN JAARSVELT ETAL 3, 3

CIRCUIT FOR MEASURING SELF-INDUCTANCES AND CAPACITANCES AND CHANGES THEREOF Filed April 14, 1959 E I F l G .7

2 INVENTOR.

MM, Z4 ZMM 9 W641 United States 3,030,576 CIRCUIT non MEASURING SELF-INDUCT- ANCES AND CAPACITANCES AND CHANGES THEREOF The invention relates to measuring circuits, in which use is made of the resonance of an oscillating circuit for purposes of measuring capacitances, self-inductances or changes thereof. When fixing the resonance point by current measuring adjustment should be effected with due observance of a maximum, which, however small the damping of the circuit may be, will always have a certain breadth, which makes for difficult adjustment.

The invention starts from the consideration that, if only the reactive part of the current would be measured, the adjustment could be much more exact, because at resonance this reactive part is zero and the rate at which the current intensity changes at a change in the frequency or at a change in the adjustment of a variable element, exactly at this point is largest.

In order to take advantage of this phenomenon according to this invention, a so-called synchronous or phase detector circuit is used, which, as is known, only gives an output voltage for that component of the input voltage, which is in phase with an auxiliary voltage which is also joined to the synchronous detector.

At resonance, the watt-component, that is the non-reactive part of the current is in phase with the alternating voltage which is supplied to the resonance circuit,'the reactive part to be measured then being zero. When there is a deviation of the resonance frequency the reactive part of the current, being the component to be measured, is no longer zero. Hereby the component has been shifted about 90 voltage. In order, therefore, to be able to represent this component which is to be measured, an auxiliary voltage must be excited which has been shifted 90 in phase relative to this alternating voltage.

This auxiliary voltage may be derived from said alternating voltage, or the current component which is to be measured may itself be shifted 90 in phase whilst using the alternating voltage which is supplied to the resonance circuit as an auxiliary voltage.

In principle the invention is applicable to any measurements in which the occurrence of resonance serves as a criterion. The invention will now be further illustrated with reference to the accompanying drawings and with the aid of the description of a special application, whereby the capacitance or the self-inductance which is to be measured is connected to the measuring circuit via a cable.

FIGURE 1 is a known circuit occurring in French Patent 1,092,928.

The FIGURES 2, 3, 4 and 5 show how the total current and the reactive part vary, when the frequency of the alternating voltage applied and the capacity of the oscillating circuit, respectively, change while the alternating voltage remains constant.

FIGURE 6 shows the measuring circuit obtained by applying the invention to the circuit of FIGURE 1.

FIGURE 7 shows vector diagrams of voltages existing in the embodiment of FIGURE 6.

In FIGURE 1 an unknown capacitance C being connected in series with a known self-inductance L is connected to one end of a cable K, which itself has a capacitance 0,, between the two conductors. The other end atent with regard to the alternating ice of this cable is connected, via a current-transformer T to a part of the secondary winding of a transformer T which part supplies the cable with the series connection with the voltage U The second part of the transformer supplies a voltage U Said current-transformer T is connected to the common point. of the two parts of the secondary winding of the transformer T The primary winding of the transformer T is connected to an alternating voltage source of low internal resistance, for instance a cathode follower circuit, and with a frequency, which is adjustable, for instance between 0.5 and 2 megacycles.

The part of the secondary winding of the transformer T which supplies the voltage U is connected via the condenser C which imitates the cable, to the end of the current-transformer T to which the cable is connected. When now the condition has been fulfilled only the current of the series resonance circuit L 0 runs through the primary winding of the transformer T A measuring instrument M, from which the intensity of the current running through the series resonance circuit can be read, is connected to the secondary winding of the transformer T The above-mentioned condition, which C must fulfil, is only correct if the cable can be represented with sulficient accuracy by a condenser, and if the leakage fluxes caused by the transformer may be neglected. When the self-inductance L is chosen judiciously and the transformer is carried out in an appropriate way there can be no objection against these conditions. If desired, the capacitance C may be replaced by a cable with properties analogous to those of the cable represented by C In order to establish a resonance, in this circuit the value of the current must be adjusted to the maximum by trimming the frequency of the voltage source.

This maximum has a certain breadth, so that an accurate adjustment tothe resonance cannot be effected. If it is required to measure small changes in capacitance or self-inductance such as occur, for instance, in condenser-microphones and invibration detectors for investigating mechanical vobriation s the frequency of the voltage source should be chosen in such a way, that the circuit operates on the flank of the resonance curve of L C According to this invention these drawbacks are avoided by measuring only the reactive part of the current which runs through the series resonance circuit. In FIGURE 2 the graph of the total current through the resonance circuit is shown as a function of the frequency, whilst in FIGURE 3 the graph of the reactive part of this current is indicated as a function of the frequency. In FIG- URE 4 and FIGURE 5 the same figures have been drawn for a constant or and a variable capacitance C. From FIGURE 3 and FIGURE 5 it is apparent that the rear tive part of the current through the series resonance circuit is zero when the circuit is in resonance, whilst the slope of this component of the current as a function of the frequency or of the capacitance is considerable. The statement of resonance as well as the observation of variations in capacitance or in self-inductance is in this way strongly improved by measuring only the reactive part of the resonance current. v

In FIGURE 6 the current-transformer T of FIGURE 1 has been replaced by a condenser C.,, which implies that the impedance should be low in respect of the impedance of the L C series circuit. The phase of the voltage which now appears on the terminals of this condenser, has been shifted with respect to the phase of the current in the series circuit.

The latter holds good when the impedance of the leakageflux of the transformer T is small in relation to the re sistance occurring when'the L G; series circuit is. in resonance.

The secondary winding of the transformer, T now consists of two equal halves. One of the halves is, via the condenser C connected with one end of the cable K, to the other end of which the series resonance circuit is connected, whereby the condenser C is at one end connected to the common point of the two halves of the secondary winding, whilst the otherend, which is connected to the cable K, is also earthed. Between this earth contact and the two ends of the secondary winding the totals and the differences of the signal voltage U and the auxiliary voltages U and U respectively, such as are usually present in a synchronous or phase detector circuit can be found. Both the voltages between the ends of the secondary winding andearth can now be amplified and rectified separately, and subsequently subtracted from each other. The voltage thus obtained is proportional to the amplitude of the reactive part of the current which runs through the series resonance circuit.

In FIGURE 7 the vector diagram or the two voltages between the ends of the secondary winding of the transformer T and earth has by way of illustration been drawn; The transformer T now also has a tertiary winding U purpose of which is to make the known compensation of the current running through the capacitance C of the cable K possible. The reference character I in the lefthand diagram'of FIG. 7 designates the vector representing the current through the series-resonant L-C circuit, which current is the same as the current through the condenser C and which current is represented as a vector leading 90 in phaseas compared to the vector representing the signal voltage U;,.

We claim:

1. Apparatus for measuring a self-inductance comprising a series resonance circuit'including said self-inductance and a standard capacitance, a pair of conductors connected to said resonant circuit, a transformer including a primary winding, a center tapped secondary winding and an auxiliary winding, a source of variable frequency alternating voltage connected to said primary winding, 2.

4 common connection connected to one of said pair of conductors, a condenser connected between said common connection and said center tap,-one end of said auxiliary winding being connected to said common connection, a condenser connecting the other end of said auxiliary winding to one end of said center tapped winding and also to the other one of said pair of conductors, a pair of amplifiers, each end of said center tapped winding being connected to the input of one of said amplifiers, a pair of rectifiers serially connected in polarity opposition between the outputs of said amplifiers and impedance means connected between said rectifiers whereby the output voltages of said rectifiers are subtracted from each other.

2. Apparatus for measuring a capacitance comprising a series resonance circuit including said capacitance and a standard inductance, a pair of conductors connected to said resonance circuit, a transformer including a primary winding, a center tapped secondary winding and an auxiliary winding, a source of variable frequency alternating voltage connecting to said primary winding, a common connection connected to one of said pair of conductors, a condenser connected between said common connection and said center tap, one end of said auxiliary winding being connected to said common connection, a condenser connecting the other end of said auxiliary wind ing to one end'of said center tapped winding and also to the other one of said pair of conductors, a pair of amplifiers, each end of said center tapped winding being connected to the input of one of said amplifiers, a pair of rectifiers serially connected in polarity opposition between the outputs of said amplifiers and impedance means connected between said rectifiers whereby the output voltages of said rectifiers are subtracted from each other.

References Cited in the file of this patent UNITED STATES PATENTS 1,708,544 Kummerer Apr. 9, 1929 1,856,707 Horton May 3, 1932 2,188,628 Freystedt Ian. 30, 1940 2,524,515 Chapman Oct. 3, 1950 2,671,198 Beverly Mar. 2, 1954 2,682,633 Woerdemann June 29, 1954 2,689,329 Zimmermann Sept. 14, 1954 2,735,064 Salzberg Feb. 14, 1956 2,759,146 Heinz Aug. 14, 1956 2,852,739 Hansen Sept. 16, 1958 2,906,950 Ichijo Sept. 29, 1959 2,939,077 Brannin May-31, 1960 FOREIGN PATENTS 723,931 Germany Aug. 13, 1942 

